An Optical Method of Strain Measurement in the Split Hopkinson Pressure Bar
Swantek, Steven David
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The split Hopkinson pressure bar (SHPB) continues to be one of the most common methods of testing materials at medium rates of strain. Elevated rates of strain, such as those found in impact and explosive applications, have been shown to induce phenomena such as strain hardening and phase transitions that can significantly affect the strength of most materials . Due to its relative simplicity and robustness, the SHPB remains one of the preferred platforms for evaluating mechanical properties of materials at rates of strain up to approximately 104 in/in-s (s-1). At the Naval Surface Warfare Center Dahlgren Division (NSWCDD), research has been conducted in which a semiconductor laser diode has been used to measure the radial strain of a plastically deforming cylindrical test specimen in the SHPB. The SHPB consists of two long, slender cylindrical bars, denoted input and output bars, that "sandwich" a cylindrical test specimen. Utilizing a high-pressure gas gun, a third cylindrical steel bar, known as the striker bar, is fired at the input bar, causing a compressive stress wave to travel through the input bar to the input bar - test specimen interface. At this interface, a portion of the stress wave propagates through the test specimen while the remainder of the pulse reflects back through the input bar as a tensile stress wave. The non-reflected portion of the stress pulse transmits through the test specimen and into the output bar causing the specimen to deform both elastically and plastically. Strain gages mounted to the input and output pressure bars measure both the incident, transmitted and reflected pulses. Specimen stress can be calculated using the transmitted strain signal while specimen strain and strain rate can be computed using the reflected strain pulse. In order to measure the specimen strain directly, a 670-nm wavelength semiconductor laser diode was affixed to the SHPB such that a vertical line of light approximately 250 micrometer (Âµm) wide was generated across the diameter of the test specimen. A collector lens located aft of the specimen was positioned to collate the light not occluded by the diameter of the specimen and refocus the light to be collected by a 25 MHz photodetector. Thus, changes in specimen diameter due to the impact event would result in more light being occluded by the specimen and less spectral energy being collected by the photodetector. The light collected by the photodetector is then converted to a voltage output before being recorded by a digital storage oscilloscope. With a known voltage-to-diameter calibration relationship, medium strain rate compressive tests were conducted to compare the optically measured strain results with the data gathered with the existing strain gages. It was found that the optical measurement system provided increased bandwidth and greater resolution than the conventional strain gage instrumentation while generating strain and strain rate results within 6.7% of corresponding strain gage data. This increased bandwidth and resolution allows the identification of both the elastic and plastic behavior of the specimen. In addition, the loading and unloading of the specimen can be clearly seen in the optical strain signal. These phenomena are evident in the peak diameter and strain achieved by the specimen, data not previously available with strain gage instrumentation. The plastic modulus, the theoretical relationship between the stress and strain in the plastic regime, also exhibits a significant increase in magnitude due to this ability to measure peak rather than average strain. Finally, by ridding the experiment of the input bar strain gage, input bar dispersion and the electrical and mechanical errors associated with the input bar strain gage were nullified. These conclusions will be validated through the presentation of several sets of experimental data correlated to data gathered previously.
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